FIG. 7 shows an example of conventional thermal printhead (see Patent Document 1 given below). The illustrated thermal printhead X includes a substrate 91 and a heating resistor element 93 extending on the substrate in the primary scanning direction. The heating resistor element 93 is covered by a protective film 94. The heading resistor element 93 is connected to an electrode 92 and another electrode (not shown) whose polarity is opposite to that of the electrode 92. When current is applied to the heating resistor element 93 via these electrodes, heat is generated. The heat is transferred to thermal paper through the protective film 94, whereby an image or letter is formed on the thermal paper.
Patent Document 1: JP-A-7-186429
Generally, to enable clear printing, the surface of thermal paper is made smooth. Examples of such surfacing techniques include the application of coating agent to thermal paper. Conventionally, however, the thermal paper having a smooth surface tends to stick to the protective film 94 when pressed against the thermal printhead X. When such a phenomenon (called “sticking”) occurs, the thermal paper cannot be smoothly slid relative to the thermal printhead X, which may result in deterioration in printing quality.
Moreover, the above-described coating agent is generally hydrophilic and tends to absorb moisture in the air. Thus, when the thermal paper is pressed against the protective film 94, the moisture which has been absorbed in the coating agent may seep out between the thermal paper and the protective film 94. Conventionally, such moisture also causes the sticking of the thermal paper to the protective film 94.